M11: The Wild Duck Cluster

I've already written about this object here, but it definitely deserves a more detailed blog post, so here we go. The 11th object on Messier's list, also referred to as the "Wild Duck Cluster", is one of the richest star clusters in our Milky Way. Its rather odd nickname was invented by Admiral Smyth in the 19th century, who saw a sort of V-shape in it, just like a flock of wild ducks. It contains some 2,900 stars, 500 of which are brighter than mag. 14, and various dark lanes which seem to divide the cluster. In spite of its considerable distance (6,100 light-years), it is one of summer's grandest objects and can already be spotted quite easily with ordinary binoculars in the inconspicuous constellation of Scutum, the shield. It's so amazingly compact that at low power it might be mistaken for a globular cluster. Increasing telescope power will reveal its true nature as a cluster of newborn stars. Being some 250 million years old, this cluster's middle-aged but considering its size and compactness there's no doubt that it will continue to resist our galaxy's gravitational pull for millions of years to come. Yet, some of the most massive stars in it have already evolved into red giant phase, meaning that they've depleted their hydrogen and are now fusing helium into heavier elements. As I've already explained, the bigger a star, the faster it will burn its hydrogen and the shorter it will live. 

The brightest star, right at the cluster's centre however... isn't a member at all. It lies 1,300 light-years closer to us! So once again you see that astronomical observations can be deceiving. 

NGC6445: Is it a box, a beetle or a crawling monster?

The peculiar planetary nebula NGC6445 goes by a lot of nicknames and most of those are not original. During the research I did for this post I came across "Box Nebula" (which it shares with NGC6309), "Crescent Nebula" (like NGC6888), "Beetle Nebula" (similar to NGC6302 - "Bug Nebula") and even "Crawling Monster Nebula". Personally, I find the first one the most convincing because rarely does nature create such angular-shaped objects.

The reason why its main ring of ionised gas and dust particles has taken on this broken rectangular form is still unclear. Planetary nebulae form under the violent radiation pressure from their dying central star and are therefore usually spherical or cylinder-shaped. Scientific research suggest that the sometimes odd shapes of older planetary nebulae are caused by the collision with slower particles that were already ejected during the star's red giant phase, long before it collapsed. The gas shell that was blown into space upon the star's death catches up with the formerly ejected shell and ploughs through it, creating a bright ring under the powerful stellar winds from the hot white dwarf. 

Our Box Nebula is, however, one of the largest and therefore one of the oldest planetaries and its central white dwarf's become extremely faint; beyond the reach of any amateur telescope. The combination of its enormous size and the cooling down of its central star have significantly reduced the impact of the stellar winds on the nebula and the ploughing has stopped. Imagine a small boat that's sailing very fast. As it's ploughing through the lake at high speed, it generates strong, geometrical ripples in the water. Now imagine that the boat suddenly stops and look at the ripples. All of a sudden they'll break up and disperse themselves randomly in all directions. Long-exposure photographs reveal a vast web of external structures and filaments, only partially visible through my telescope, which extend up to two light-years into space.

We can conclude that this nebula has entered the last phase of its life. As it slowly expands further and as the central star keeps cooling down, also the nebula will cool down and eventually dissolve completely.

 

 

M27: Her Royal Majesty the Dumbbell Nebula

M27 is the brightest of the so-called planetary nebulae in the sky. The classification "planetary" was derived from the fact that these nebulae often appear like little disks, much like a planet. In reality they're huge shells of gas that were expelled by a dying star.

Right at first glance it's obvious where this particular nebula got its "dumbbell" nickname from, isn't it? Actually, we see this nebula from its equatorial plane. When viewed from its poles it would probably appear ring-shaped, much like the Ring Nebula in Lyra. As I've explained before, it's best to imagine these older, more developed planetary nebulae as an apple without a core. In this case the bright, inner gas shells that were expelled during the collapse of the central star, have caught up with the thinner external shell that already formed before the star exhaled its dying breath. Together they're expanding at 31km/s until they'll dissolve into space. The age estimates vary greatly, between 4,000 and 15,000 years, but the most recent spectroscopic analysis suggests an age of somewhere in between: 9,800 years. There's also been a lot of controversy regarding the Dumbbell’s distance but 1,300 light-years seems to be the most recent consensus. From this distance, the nebula’s a full light-year across.

In order to find the reason why this nebula became so big and bright, we have to examine its central star which is already visible in small telescopes. This star used to be a giant containing as much as ten solar masses. It was big, but not quite big enough to explode as a supernova. Even now, the dying stellar core has a diameter of 70,000km and it still contains 60% of our Sun’s mass, making it the biggest white dwarf known. With an extremely hot surface temperature of 85,000°C, it heats up the vast gas clouds so much that they start to emit a bluish-green light.

Today, the Dumbbell’s one of the brightest and most popular objects in the northern skies, extending almost half of the diameter of a full Moon and easily visible through a pair of binoculars. It’s a privilege to be able to observe it with an 18” binoscope under a reasonably dark sky.

Me 1-1: A difficult name doesn't necessarily mean a difficult object

When I was a kid, I had this big poster of the stars above my bed. It was my perennial source of inspiration and I was constantly scanning it in search for new objects. Being the proud owner of a modest 60mm refractor, which nevertheless was considered a medium-type telescope at the time, I knew that I didn’t have to put the more daring objects on my observing list. So I stuck to the classic Messiers and I felt proud to have seen a great many of them. But on the chart there were also other objects, which had the scary “NGC” denominator, followed by a four-digit number. At first I sort of neglected them in the knowledge that they would turn out to be invisible in my telescope. Also my friends at the Antwerp observatory talked with great apprehension about the NGCs and so I never even bothered to try them, apart from those which I didn’t know were NGCs such as the Double Cluster in Perseus, which is already visible to the naked eye. 

And still... there was one that fascinated me. Right at the heart of my stellar map, near the celestial North Pole, there was this tiny little nebula denominated NGC6543. Was it the number that sounded like a countdown? I don’t know what it was exactly that eventually pushed me to give it a try, regardless of its so-called impossibility status. Surprisingly I didn’t have to look very long in order to find it. Yes, it was small, very small in my humble telescope, but it was unmistakingly there and quite bright too! The next Saturday I couldn’t wait to go back to the observatory to tell all of my astronomy-friends about my observation and for many years to come I felt so proud of this achievement. It was the start of my unstoppable passion for the real faint fuzzies, the ones which others deem impossible, the weird ones, the never-heard-of ones. Many of those reveal themselves to be extremely difficult indeed. Others, like exotic NGC6543 in my 60mm scope, turn out to be remarkably easy. 

Take this little bugger for example. It carries the name Merrill 1-1 (or Me 1-1 in short) after its discoverer. I have to admit that I only came across this one by accident and I suppose that only freaks will know about its existence. But when I pointed my binoscope at it, it appeared surprisingly bright. Yes, it’s tiny and I advise you to use an OIII filter when trying to locate it because in small telescopes or at low power it will look almost stellar. But once you’ve found it, try to push magnifications as much as you can and perhaps this little planetary nebula will reveal as much to you as it did to me. At first glance it’s just a tiny patch and not very interesting as such. But the reason why it’s so small is because it lies incredibly far away from us: 19,000 light-years if you please! Imagine that! This almost certainly means that at the time of writing this planetary nebula won’t be there anymore. It will have completely dissolved into space! It’s just that the light from it being blown up and scattered hasn’t reached us yet. 

Another puzzling thing to which scientists have only recently found the answer is its central star (which was completely invisible to me by the way). This central star’s quite cold with a surface temperature of only 4,300°C (against e.g. 5,500°C for our Sun) and therefore it remained a mystery how such a cold star could heat up the nebula enough for it to emit light. The central star’s also displaced a great distance from the nebula’s centre, by over 10 times the distance to Pluto. A spectroscopic analysis in 2008 revealed, however, that this star isn’t the cause but that a small companion star is. It is a double star, very much like Ras Algethi, the smaller member of which died and released its atmosphere into space.

From our point of view, we see this nebula edge-on and it’s supposed to have a ring-like shape when seen top-down. All that I could see was an elongated sort of box, but what struck me in particular were two cavities on opposite sides. I don’t know if I’ve observed this correctly since it was so small, so any confirmation would be greatly appreciated.    

 

Aquila's double cluster (part 2)

About half a degree to the west of NGC6756 lies this open cluster, called NGC6755. Both are definitely one of these very rare "real" double clusters, i.e. which share the same origin. They lie more or less at the same distance (5,000 lightyears) and are just as young (only 8 million years old). Intrinsically they look very similar i.e. most of the stars in both clusters are more or less of the same brightness. But that's where the likeness ends because NGC6755's a lot bigger and richer (80 confirmed members against 40 or so for NGC6756). Then again, NGC6756's a lot more compact and appears more like a faint globular in small to medium telescopes or at low power. For its bigger companion I obviously used lower power in order to fit all of it nicely in the field of view.

Another thing that struck me during my observation, was that NGC6755 appeared somewhat split in two. After doing some research there does seem to be a dark dustlane blocking the light of some of the stars, cutting across the field from top-right to bottom-left. 

Again I must warn you that appearances may be quite deceptive. Stars that seem to belong together from our point of view may be hundreds or even thousands of lightyears apart. The orangy star on the right, for example, is very old and most certainly isn't part of the cluster. The same goes for the somewhat yellowish star below the centre of the field. Unfortunately I wasn't able to find any distance data for these two stars, but I can confirm that the bright star near the top-left edge is a giant that lies a thousand lightyears further away from us. 

 

Aquila's double cluster (part 1)

Stars are born together in huge clouds of gas, sometimes by the hundreds such as in the Orion nebula. These young and hot stars burn fast and live a wild life, like adolescents, emitting violent radiation which not only illuminates the gas cloud from which they were born, but makes it emit light as well. Eventually the compound radiation from all these young stars will blow the nebula to pieces and the gas disperses into space, leaving only a cluster of stars. Scientists estimate that only 10% of the original gas cloud condenses into stars and that the rest blows away into space, possibly to coagulate into a nebula again when there's enough matter around to let gravity do its work. 

This little gem, denominated NGC6756, is a cluster of young stars in the constellation of Aquila, the eagle, not more than 8 million years old. It lies 5,000 lightyears away and so it isn't the brightest of objects. Actually, the brighter foreground stars, the brightest of which is of magnitude 11,9,  don't belong to the cluster at all but are dwelling much closer to us. The stars of this cluster are therefore quite faint, from mag. 14 down to mag. 18, and are difficult to resolve in smaller telescopes, also because the cluster's still fairly compact. Yet, this makes this particular cluster such an interesting object because even if it doesn't look stellar in a small instrument, it will appear as an attractive nebulous patch. With the binoscope however I had no trouble at all identifying all 40 or so members.

The most interesting thing about this little cluster is that it's one half of a rare double cluster. Although we've identified well over 1,100 of these star clusters in our Milky Way, only a handful of them are double, meaning that two separate clusters share the same origin. The most famous of those if of course the Double Cluster in Perseus, but also NGC1807-1817 in Taurus are two clusters born from the same cloud. As with most clusters, the stars of NGC6756 will eventually drift apart under the pull of our galaxy and most of them will lead solitary lives.

 

Very old, but still alive and kicking

Globular clusters are among the oldest entities in the universe. These strange and extremely dense balls of stars are generally older than the galaxies they accompany and some have emerged only just after the Big Bang. With "just" I intend hundreds of millions of years of course. At first sight they all seem alike, but when you take a closer look you'll notice that they're surprisingly different from one another. Take extremely compact M15, for example, and compare it to majestically large and irregular M5. There's NGC2419, so far away that it only marginally feels the gravitational pull of our Milky Way and NGC5466 that's literally torn to bits by it.  There are 150 to 160 of these globulars known to orbit our Milky Way but other galaxies such as giant M87 dominate over 12,000 of them!

And then there are the survivors, the oldest of them all. NGC6426 is, with a distance of almost 70,000 lightyears, quite far away from us. It's so far that you need a sizeable telescope and a good-quality sky in order to resolve some stars in it. Yet, it's a very interesting globular cluster because it's one of the oldest there is. We can tell because this cluster has an unusually low metal content. With "metal" astronomers mean everything heavier than hydrogen or helium. When the universe came to be 13.7 billion years ago, the first chemical element that arose was hydrogen of course, quickly followed by helium which was created through the fusion of hydrogen in the very first stars. But for anything heavier, we had to wait until these first stars ran out of hydrogen and started fusing helium into more complex elements such as carbon, nitrogen and oxygen. This took hundreds of millions to even billions of years. The stars in globular clusters, on the other hand, are extremely slow burners and although very old most of them are still in their hydrogen-fusing phase. The fact that the stars of NGC6426 contain such an extremely low level of "metals" indicates that this cluster was formed very early in the evolution of our Universe.  

Two old Suns

Next time you're observing the Turtle nebula in Hercules' belly, point your telescope slightly to the south and you'll bump into this lovely pair. STF2094 is a couple of greenish-yellow giant stars over 500 lightyears away. The term "giant" seems somewhat overrated at first sight since the biggest of the two's just 3,8 times the diameter of our Sun and contains only 1,3 solar masses. The giant classification in this case doesn't concern the star's mass or diameter, but rather its luminosity and the phase of stellar evolution. Originally these stars were not much unlike our Sun and they used to be much dimmer too. Then, suddenly, they ran out of hydrogen and it's exactly the change into a helium-fusing red giant that we're witnessing here, something that will also happen to our Sun in over 5 billion years. The transition into a cool, red giant isn't instantaneous and initially the star in question will not only brighten but also heat up slightly. The surface temperature of the two stars on my sketch measures almost 1.000°C hotter than our Sun. A famous example of a star in this phase of evolution is Capella, the brightest star of the winter constellation of Auriga and the sixth brightest star in our sky. When the star expands further, its surface will gradually cool down and the star becomes an old, red giant. 

The couple on my sketch is therefore quite interesting because they used to be so similar to our Sun and because they seem so close to one another. Well, you have to interpret the word "close" in astronomical terms because in reality these two stars are 171 times the distance between the Earth and the Sun apart, or almost six times the distance to Pluto! And yet, even from this distance, these stars would appear hundreds of times brighter than a full Moon to each other!

This double star represents a bit of a challenge for small to medium telescopes and you need a good-quality sky in order to separate them. Then again, that's precisely what double star fanatics are looking for or course. 

But... wait a minute! Before you go, focus on the upper-right corner of the field of view. Isn't there... something... ? Yes! The faint patch you may notice is a spiral galaxy (catalogued as UGC10525), quite similar to our Milky Way, but very distant. It lies 430 million lightyears away...

 

The Phantom Streak

In a recent post, I showed you an image of a dying star. Fusion of helium into heavier elements such as oxygen and carbon became critically unstable and the star collapsed under its own gravity, blowing away its entire atmosphere in the process. The expelled gas cloud's not become a so-called planetary nebula yet i this case because it isn't emitting light on its own. For this we have to fast-forward one or two thousand years until the remaining, super-hot nucleus of the dead star heats up the expanding gas bubble so much that it ionises. This is exactly what I'd like to show you here. NGC6741, aka the "Phantom Streak" nebula in Aquila, is an extremely young planetary, just marginally older than the Footprint of my former post. The difference is that the Phantom Streak has begun to emit light on its own and therefore it truly deserves to be categorised as a planetary nebula, whereas the Footprint isn't quite there yet. Being so young the Phantom Streak's incredibly tiny and needs a lot of telescope power to be recognisable as a nebula and not a star. On my sketch you can clearly see the tiny gas envelope with its rectangular shape. As I've explained before, stars are less dense at the poles, allowing the gas to escape more easily there. For this reason planetary nebulae often take on an elongated or even cylindrical shape such as the famous Ring nebula. If you look at the Saturn nebula, you'll also note the highly elongated internal structure. 

So how did planetary nebulae come by their "planetary" nickname you might ask? Yes, they got it because they usually do display a round, sort of planetary shape. This outer sphere, however, was already expelled earlier, thousands of years before the star exhaled its dying breath. Stars close to the end of their life swell significantly and become unstable. They grow under the pressure of the complex fusion process in their core until they reach the point that they've become too big and cool down. Gravity takes over and the star contracts again, heating up the star's surface until fusion shifts to a higher gear, and so on. In this unstable period the star already loses a large part of its atmosphere. Sometimes a very big star may lose so much of it that what remains contracts up to the point that fusion becomes stable again and the star's surface may even become extremely hot. The Thor's Helmet nebula is such an example, a giant star that's got rid of a large part of its atmosphere that's being hurled into space under the fierce radiation of the remaining very hot star. Eventually, in the case of a normal planetary nebula, the gas bubble that's expelled by the sudden death of the central star will catch up with the external gas shell and they'll dissolve into space together. 

As regards to our Phantom Streak, the external shell isn't visible to amateur telescopes for the time being since it hasn't been heated up sufficiently. But in another one to two thousand years it will look very much like the Saturn nebula or similar planetaries.

Going their own way

Stars are usually born together out of giant hydrogen clouds like the Orion Nebula. The gas cloud condenses under its own gravity and spawns dozens to even hundreds of stars until it's spent and the remaining gas dissolves into space. What's left is a cluster of stars that during their childhood remain together, bound by their mutual gravity. But after a while, the much stronger gravitational force of our galaxy will break their bond and disperse the stars, like going through a blob of paint with a thick brush. The stars will then each go their own way, just like children that've grown up will leave their parents' home to build a life of their own. A very fine example of this is probably the most famous constellation of them all: Ursa Major, the "great bear" or "big dipper". All of the stars in the "big dipper", apart from two (Dubhe and Alkaid), share the same origin and they were born together some 500 million years ago, together with a few dozen others among which Alpha Coronae Borealis (Gemma or Alphecca), Beta Aurigae (Menkalinan) and Delta Aquarii (Scheat).

What I'm showing you on this sketch is a star cluster containing about 30 members, NGC6633, that's breaking up. The young, hot stars are leaving the nest and will start their individual journeys through space very soon. They've already moved so far away from each other that with my big telescope I was almost looking through them. Definitely this lovely cluster's best enjoyed with smaller instruments at low power or even with a pair of binoculars... for as long as we still can. Within another 100-200 million years the stars will be scattered and the cluster will be no more.

Antennae

Spring is nearing its end and so we're saying goodbye to the galaxy season. In order to go out with a bang, I present you one of Spring's classics: the famous "Antennae" galaxies in the obscure constellation of Corvus, the crow. Frustration will be the share of observers in the more northern latitudes because this object appears not very high above the horizon to them. Even from my home in Northern Italy I had to point my binoscope fairly low, into the hazy glow above the mountains. Yet, I've never seen this pair of colliding galaxies like this before. Simply amazing! 

NGC4038 and NGC4039 are indeed two spiral galaxies that are crashing into each other right as we speak! Or make that 63 million year ago because that's how long it takes for their light to reach us. A billion years ago, they were still two separate galaxies, each minding its own business. But alas, gravity condemned them to a dramatic fate. 600 million years ago, they started to collide and NGC4039 (the somewhat smaller, upper half on my sketch) literally passed through its counterpart whereby both galaxies were severely disrupted. Both galaxies released long tails of stars in the process, which were invisible to me but which show well on long-exposure photographs - hence the nickname "Antennae". These star trails extend some 360,000 lightyears into space! 

Within another 400 million years the Antennae's nuclei will collide into a single, supermassive black hole. Simulations suggest that the galaxies will eventually melt into a big, elliptical galaxy, much like M87. For the time being, the collision's triggering an incredible burst of activity and star formation, as you can guess from the many complex and bright internal structures.

This is also the fate that awaits our own Milky Way in 5 billion years, when we'll crash into the Andromeda Galaxy. But don't worry, this doesn't mean the end. Even though galaxies contain hundreds of billions of stars, they're mostly void and it's not likely that the stars themselves crash into each other. Think of the enormous distance between our Solar System and the nearest stars! No, the Earth will die around approximately the same time - supposing that we, stupid human beings, haven't cocked up our planet before - but not because of the collision with Andromeda. In 5,4 billion years from now, our Sun will run out of hydrogen and will enter its red giant phase, fusing helium into heavier elements. This will cause our Sun to grow considerably; so much that Mercury and Venus will be swallowed up and the Sun's outer atmosphere might even reach the Earth. So we will get scorched or our planet may even evaporate altogether. Suddenly a collision with Andromeda seems somewhat less worrying, doesn't it?

Racquetballs are blue

In the same part of the sky where you can find the Turtle Nebula, you may bump into this little bugger. At low power you might mistake it for an unusually blue star because it's so tiny, but pushing magnifications will reveal a surprising amount of detail. It's still a very young planetary nebula, not more than a few thousand years old, slightly more evolved than the Footprint Nebula. If the ancient Egyptians had a telescope, they would only have seen a reddish little star. Compared to the Footprint, the Blue Racquetball's central star has completely shed its atmosphere and the exposed, extremely hot core has started to heat up the surrounding gas shells up to the point that they're emitting light. You can clearly see the faint, external shell of gas that had already been expelled earlier during the last and very unstable phase of the star's life. The very complex inner structures consist of the star's atmosphere that's now whirling ever further into space, blown away by the powerful winds that the star's collapse generated. The best guess for this nebula's distance is 4,900 lightyears which would mean that the nebula's a third of a lightyear across. This is still fairly small and that's why we see it so brightly. Within another couple of thousand years, the gas cloud will have grown over a lightyear across and the dispersed gas will dim, eventually becoming invisible.